<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        #box {
            margin: 100px auto;
            width: 100px;
            height: 100px;
            border: 1px solid #000;
            text-align: center;
            line-height: 100px;
        }

        #btn {
            width: 50px;
            height: 20px;
        }
    </style>
</head>

<body>
    <div id="box">
        <button id="btn">按钮</button>
    </div>

</body>
<script>
    var btn = document.querySelector('#btn');
    btn.addEventListener('click', function () {
        this.style.color = 'red'
        this.style.borderRadius = '25px'
    })
</script>

</html>